In an existing Long Term Evolution (LTE) system, there is only one carrier with a maximum bandwidth of 20 MHz available in a cell. In a Long Term Evolution-Advanced (LTE-A) system, peak rates of the LTE-A system have been improved so greatly over the LTE system that there are a required downlink rate up to 1 Gbps and uplink rate up to 500 Mbps of the LTE-A system. Obviously such a demand has not been satisfied by the bandwidth of 20 MHz. Carrier Aggregation (CA) has been introduced to enable the LTE-A system to satisfy the demand.
Carrier aggregation refers to that the ability of a user equipment to operate concurrently in a plurality of cells which can be consecutive or inconsecutive in the frequency domain and which may have the same or different bandwidths. A maximum bandwidth of each cell is limited to 20 MHz for compatibility with the LTE system. At present it is generally recognized that the number of cells aggregated for a user equipment is 5 at most. Moreover the LTE-A system further categorizes the cells aggregated for the user equipment as follows dependent on different functions that may be supported in the different cells:
A Primary Cell (PCell), where only one of the cells aggregated for the user equipment is defined as a PCell which is selected by an eNB and configured in Radio Resource Control (RRC) signaling to the user equipment and only in which a Physical Uplink Control Channel (PUCCH) is configured.
A Secondary Cell (SCell), where all the other cells aggregated for the user equipment than the PCell are SCells.
Power Headroom Reporting (PHR) defined in the LTE-A Release 10 (R10) requires a report of Power Headroom (PH) information in configured and active cells, that is, PH information in all the configured and active cells is packaged and reported together once is PHR is triggered.
In the R10, only intra-band Carrier Aggregation (intra-band CA) is supported in the uplink, so the same Time Division Duplex (TDD) configuration is sure to be used for all the uplink carriers aggregated for a user equipment. In an R11 system, aggregation of cells from different band (inter-band CA) will be introduced, so a frequency band aggregated in the LTE-A system may be shared with or adjacent to another system (e.g., the LTE system), for example, three cells including Cell 1, Cell 2 and Cell 3 are aggregated for an LTE-A user equipment as illustrated in FIG. 1.A, where the same Band 1 is used for the Cell 1 and the Cell 2, and a Band 2 is used for the cell 3. In order to avoid uplink/downlink cross interference between TDD systems, TDD Uplink/Downlink (UL/DL) configurations that can coexist, i.e., configurations without UL/DL cross interference, meaning the same TDD UL/DL configuration for the LTE system, shall be used for the Band 1 and a 3G/LTE TDD Band A. TDD UL/DL configurations that can coexist shall be used for the Band 2 and a 3G/LTE TDD Band B. If different TDD UL/DL configurations are used for the Band A and the Band B, then TDD UL/DL configurations used for the Band 1 and the Band 2 are also different.
For the R10, only intra-band aggregation is supported in the uplink, so all the cells are either an uplink subframe or a downlink subframe at the same instant for the TDD system, and thus the user equipment can report PH information in all the configured and active cells in an uplink subframe once PH reporting is triggered so that the eNB obtains the PH information of the user equipment as a whole.
For the R11, due to the introduction of inter-band aggregation and different TDD UL/DL configurations, subframe states of different cells may be different as illustrated in FIG. 1B. Assumed TDD UL/DL configurations 0 and 1 are adopted respectively for the Cell 1 and the Cell 2, for an instant T1, if PH reporting is triggered in the Cell 1 and PH information is currently reported only for a configured and active uplink subframe, then the user equipment will report only PH information of the Cell 1 at the instant T1, and the PH information can not reflect a power allocation condition of the user equipment in the case that both the Cell 1 and the Cell 2 are uplink subframes, so when the Cell 1 and the Cell 2 are uplink subframes in all the instants T2, T3 and T4, the eNB can not perform UL scheduling of the instants T2, T3 and T4 with use of the PH information obtained at the instant T1.
In summary, PH information is currently reported for only an uplink subframe, so the existing PHR mechanism discourages subsequent scheduling in the case that there are different TDD UL/DL configurations for a plurality of configured and active cells aggregated for a user equipment.